1. Field of the Invention
The present invention relates to a lead frame for a semiconductor device and in particular to a lead frame for LOC used in a semiconductor which is subjected to mold packaging in an LOC structure.
2. Description of the Related Art
Lead frames for semiconductor devices having an LOC (lead on chip) structure or the like, which can realize high density assembly, are known in the art. One of them is a lead frame used in a system where a high heat-resistant insulating film coated with a thermoplastic or thermosetting adhesive layer is applied to one side or both sides of the lead frame and a semiconductor chip is mounted on the high heat-resistant insulating film under heating or under pressure.
For the lead frame in this system, it is common practice to use a polyimide film as the high heat-resistant insulating film and, in the application of the film to the lead frame, die stamping application is used. Specifically, a film in a reel form is stamped by means of a die into a desired shape and then applied to the lead frame under heating or under pressure.
According to this method, the film is stamped by means of a die, and the stamped film is then applied to the lead frame in its desired position. Therefore, the amount of the tape used is large, resulting in increased cost. Further, since stamping of the film produces a waste film, the material is wasted. Furthermore, absorption of moisture into the polyimide film leads to a fear of package cracks being created.
In order to eliminate such inconvenience, a method has been proposed which comprises: coating an adhesive onto a lead frame in its area where a semiconductor device is to be mounted; and jointing (bonding) the semiconductor device to the lead frame with the aid of this adhesive.
For the coating of the adhesive, & device comprising a combination of an X-Y robot with a dispenser is generally used to coat the adhesive onto a lead frame in its area where a semiconductor device is to be mounted (for example, a front end of an inner lead). In particular, when an adhesive is applied to the front end of the lead, a spot coating method is used. In this method, what is required is only to coat a required amount of the adhesive onto the lead frame. Therefore, no waste material is produced, and no expensive die is required, advantageously resulting in reduced production cost.
A varnish adhesive (for example, a solution of an adhesive resin in a solvent) is coated by ejecting the adhesive through a capillary needle (or nozzle) by means of air pressure onto a lead frame in its predetermined area while moving the needle (or nozzle) on the lead frame.
FIG. 1 is a plan view of a conventional lead frame coated with an insulating adhesive. A lead frame 1 comprises: inner leads 2 concentrated in a semiconductor chip mounting region and provided so as to face each other from both directions; outer leads 3 connected respectively to the inner leads 2 and provided parallel to one another; a tie bar 4, provided on the outer leads 3 in their portions to be exposed after plastic molding, so as to connect the leads to one another; and a frame section 5 provided on both sides of the frame so as to support the tie bar 4.
In a lead frame having such a structure, an insulating adhesive 6 is coated by the above method onto the front end of the inner leads 2. A semiconductor chip (not shown) is mounted inside a chip shape line 7 indicated in FIG. 1. The semiconductor chip, when mounted, is bonded to the insulating adhesive 6 on the leads under heating or under pressure.
According to the lead frame using the conventional adhesive coating means, a varnish adhesive is placed in a container and ejected from the container with the aid of air pressure. In this case, a time lag is created between the initiation of the pressurization and the incitation of the ejection. This appears as a change in adhesive in the container. Further, when the application of air pressure is stopped, the ejection of the adhesive is not immediately stopped due to the influence of the residual pressure or the like in the container. For this reason, the coverage of the adhesive is instable at the time of the initiation of the costing and the termination of the coating. A large variation in coverage of the adhesive adversely affects the fixation of the semiconductor chip and wire bonding.